Such a tray is known from U.S. Pat. No. 4,550,000. This publication describes a vertical gas-liquid contacting column provided with a plurality of trays. These trays comprise of a rectangular downcomer having inclined side walls and liquid discharge openings in the closed lower end of the downcomer. According to this publication the aggregate area of the liquid discharge openings in the lower end of the downcomer should be sufficient for discharging all the liquid which flows down the column at the intended liquid loading and should be restricted in relation with the horizontal cross-sectional area of this lower end of the downcomer so as to maintain in the downcomer a column of liquid which exerts at the liquid discharge openings a hydrostatic head which is sufficient to prevent gas from entering the downcomer from below.
In the above publication no further reference is given to the actual slope or cross-sectional area in the lower end of the downcomer. According to general textbooks like “Distillation Operation” by Henry Z. Kister, McGraw-Hill Inc, 1990, pages 173-175, the ratio of the cross-sectional area at tray level to the cross-sectional area at the bottom of a sloped downcomer is between 1.5 and 2.0 and typically 1.7.
U.S. Pat. No. 5,407,605 describes a tray provided with rectangular downcomers having no cross-sectional area at the lower end. Instead the downcomer walls meet at their lower end and side-wards directed liquid discharge openings are present in the lower end of the downcomer walls. According to this publication the horizontal velocity of the liquid being discharged from these openings is beneficial because it helps spreading the liquid over any packing which may be located below the tray.
When trying to increase the liquid and/or gas flows of gas-liquid contacting columns, which are provided with the above described trays of U.S. Pat. No. 4,550,000, a maximum load will be observed. Higher loads will result in that the column fails to function as a liquid-gas contactor or separator due to a phenomena known as flooding. Flooding is described as excessive accumulation of liquid inside the column. The well known flooding mechanisms are downcomer back-up, jet flooding and downcomer choking. These mechanisms are described in the above referred to general textbook “Distillation Design”, Henry Z. Kister, McGraw-Hill Inc, 1992, page 267-291. According to this publication downcomer back-up is due to a build-up of liquid inside the downcomer causing the liquid to back-up on the tray leading to liquid accumulation on that tray. The liquid height in the downcomer is determined by the tray pressure drop, liquid height on the tray and frictional losses in the downcomer and downcomer slot area. Jet flooding or entrainment flooding is caused by a too high gas velocity leading to the entrainment of liquid, either by droplets or froth, to the tray above. The liquid will accumulate and leads to flooding. Downcomer choking is caused by a too high aerated liquid velocity in the downcomer. At a certain velocity the friction losses in the downcomer and downcomer entrance become excessive, and the frothy gas-liquid mixture cannot be transported to the tray below, causing liquid accumulation on the tray. With the term froth is to be understood any gas-liquid mixture present on the tray not depending on any flow regime.